1. Field of the Invention
The invention relates to a blast cleaning machine where workpieces are cleaned by the impingement of high speed abrasive particles and, more particularly, to a conveyor for moving workpieces through such a blast cleaning machine.
2. Description of the Prior Art
Blast cleaning machines for cleaning workpieces by the impingement of high speed abrasive particles usually have a housing which defines a chamber. One or more centrifugal throwing wheels are carried by the housing and project streams of high speed abrasive particles into the chamber. Workpieces being moved through the chamber will be impacted by the abrasive particles and will be cleaned of sand, rust, scale, and other debris.
A persistent problem with blast cleaning machines is the means by which workpieces are moved through the chamber. A particularly effective blast cleaning machine is shown in U.S. Pat. No. 3,903,652, issued Sept. 9, 1975 to D. L. Baughman and J. H. Carpenter, Jr. (hereafter "the Axi-Flow patent"). In the Axi-Flow patent, workpieces are advanced successively through an elongate barrel having a skeletal wall construction open at both ends. The barrel is long enough to permit the longitudinal passage of a single line of workpieces through the barrel. As workpieces are advanced through the barrel, the barrel is rotated. A plurality of abrasive throwing wheels are positioned adjacent the barrel to project abrasive particles at high speed through the skeletal walls onto workpieces being advanced through the barrel. By appropriate control of the speed of travel of the workpieces through the barrel, and by rotating the barrel at appropriate speeds (or not rotating it at certain times), almost all surfaces of the workpieces can be cleaned quite effectively.
A drawback of the Axi-Flow patent is that the workpieces are pushed through the barrel in end-to-end contact. Although the Axi-Flow patent sets forth several techniques by which the ends of workpieces can be cleaned better, such as by casting spacers in place at the ends of the castings or by orienting alternate workpieces at right angles to adjacent workpieces, no totally effectively automatic technique is available to clean the ends of the workpieces.
Other techniques are known for conveying workpieces through blast cleaning machines, although most of these known techniques have various drawbacks. For example, tumbling barrel-type devices are known wherein a plurality of workpieces are tumbled and conveyed en masse in the path of high speed abrasive particles. Representative tumbling barrel-type devices are shown in U.S. Pat. No. 1,581,045, U.S. Pat. No. 3,693,296, U.S. Pat. No. 4,218,854, and U.S. Pat. No. 4,368,599. A problem with the referenced devices is that workpieces can be nicked during the cleaning process due to workpieces impacting one another as tumbling occurs. Another difficulty with these devices is that some workpieces can be buried in the mass of workpieces, and thereby shielded from the blast stream. It thus is possible that the workpieces can be cleaned inefficiently or portions of the workpieces may not be cleaned at all.
Table-type and belt-type machines are known wherein workpieces can be cleaned on one side during their passage through the machines, but the parts must be manually turned over and passed through the machines a second time in order to clean the other side of the workpieces. In some cases, the workpieces need to be turned end-for-end for yet an additional pass through the machines. Even if an automatic turnover device is employed, a plurality of blast wheels must be employed in order to obtain complete coverage. Table-type and belt-type machines clean relatively inefficiently because the head and tail portions of the stream of abrasive particles are not applied to the workpieces being cleaned.